1. Field of the Invention
The present invention relates to an electrochemical sensor comprising a work electrode and a reference electrode, said sensor adapted to measure the characteristics of a liquid under test, based on the relative electric potential difference and electric current between the work electrode and the reference electrode both immersed in the liquid under test as a pair, said work electrode comprising a substrate formed from platinum, gold, platinum black, etc., and said reference electrode comprising a substrate formed from silver coated with silver oxide film or a substrate formed from gold.
2. Prior Art
In a conventional electrochemical sensor, for example, a hydrogen ion concentration (hereafter referred to as “PH”) sensor, a work electrode is configured in such manner that a tube filled with a solution at the known PH value as the internal liquid is provided, a substrate forming the work electrode is immersed in the solution within the tube, and one end of the tube positioned adjacent a liquid under test is provided with a thin glass film having hydrogen ion selection capability via which the internal liquid is made contact with the liquid under test. On the other hand, a reference electrode is configured in such manner that a tube filled with a salt bridge solution at neutral ion concentration as the internal reference liquid is provided, a substrate forming the reference electrode is immersed in the solution within the tube, and a portion of the tube immersed in the solution under the test at the time of measurement is provided with a liquid communication section formed from porous ceramic, porous resin, etc., via which the internal reference liquid is made contact with the liquid under test.
One such conventional configuration can be found, for example, in Japanese Patent Laid-Open No. 11-258197 (hereafter referred to as “Patent Literature 1”) and it is especially illustrated in FIG. 5 of this patent literature. Referring to correspondence between the names of the components in FIG. 5 of Patent Literature 1 and those of the PH sensor as described above, a glass electrode “a” is the work electrode, a glass electrode body “c” is the tube of the work electrode, a glass electrode internal liquid “f” is the internal liquid, a glass electrode internal pole “e” is the substrate of the work electrode, a response glass “d” is the thin glass film, a comparison electrode “b” is the reference electrode, a comparison electrode body “h” is the tube of the reference electrode, a comparison electrode internal liquid “j” is the internal reference liquid, a comparison electrode internal pole “i” is the substrate of the reference electrode, and a liquid communication member “m” is the liquid communication section.
Furthermore, in an oxidation-reduction potential (hereafter referred to as “ORP”) sensor, a work electrode is configured such that a substrate forming it is directly immersed in a liquid under test, which obviates the need for an internal liquid, a glass tube, etc. The reference electrode is, however, configured in the same manner as the PH sensor, as described above.
Moreover, in a PH meter and an ORP meter incorporating the electrochemical sensor configured as described above, an amplifier having higher input impedance is included therein.
Japanese Patent Laid-Open No. 11-258197 (Patent Literature 1) is incorporated herein by reference.
However, the previous PH sensor described above has been suffered from many deficiencies: the sensor is difficult to handle or manufacture because of liquid such as the internal liquid, the internal reference liquid, etc., involved therein; and the sensor is limited in miniaturization of the tube continuously filled with the liquid. In addition, because of necessity to provide one end of the tube with the liquid communication section formed from thin glass film or ceramic, the glass tube capable of connecting under the sintering has been used, but it was very difficult to handle. For the work electrode the thin glass film in contact with the liquid under test is significantly small in thickness and is likely to be broken. For the reference electrode the internal reference liquid tends to slightly leak out via the liquid communication section. Therefore, if no action taken, some crystal of the internal reference liquid with moisture vaporized is deposited on the surface of the liquid communication section, which necessitates cleaning of the surface of the liquid communication section to which the liquid under test is contact, prior to the measurement. Furthermore, in order to replenish the internal reference liquid by an amount of leakage it was necessary to provide any additional source of the internal reference liquid for replenishing. In addition, there was possibility in the liquid communication section that the liquid under test may reversely flow to lower the concentration of the internal reference liquid and to cause any change in potential between the liquids, which leads to measurement error.
The ORP sensor is also difficult to handle and has a limitation in miniaturization because of liquid catalyst used in the reference electrode.
Because the electrode formed from the thin glass film or the liquid catalyst has an increased internal resistance it is necessary to use an amplifier having higher input impedance for increasing the voltage revel for measurement, which adds the cost of the device.
In view of the above an object of the present invention is to provide an electrochemical sensor that obviates the need for the internal liquid by solidifying the electrode using various types of thin films, that is easy to handle, and that has capability of miniaturization.